Tool to change Package/Texture... how? (EXPLANATIONS)

Being new to Lua, I have been trying to use a trick of learning portions of the language from the tutorials as well as pulling from created gear, such as RobloTim's RobloHunk Potion, and have been able to alter the code to involve a specific hat, specific packages, and the whole ordeal...

...but what am I missing to add to the RobloTim's RobloHunk Potion to include custom decals? I would prefer creating a tool that changes the user's package, along with using a custom decal from my inventory (asset ID; ROBLOX Image/Texture/Decal) that also applies to the package and the hat(s)... which I will be figuring out as well.

I'm not looking for handouts on this, I'm expecting an explanation on this. If you could teach me how to do this, that would be much preferred.